Process for the recovery of hydrocarbons



April 1937- J. K. ROBERTS E'l AL 2,077,344

PROCESS FOR THE RECOVERY OF HYDROCARBONS Filed Nov. 16, 1934 ATTORNEYS Patented Apr. 13, 1937 V UNITED STATES PATENT OFFICE PROCESS FOR'T'HE RECOVERY OF HYDROCARBONS ration of Indiana Application November 16, 1934, Serial No. 753,290

6 Claims.

Our invention relates to a system for obtainin stabilized gasoline from gases and vapors. Such gases or vapors may comprise a mixture of hydrocarbons between the range of one to eight carbon .-molecules obtained either from natural gas or a distillation or cracking system. In the latter case, of course, beside saturated compounds the corresponding olefines or cyclic compounds might be present.

Various systems have been devised for separating a mixture of gaseous and vaporous lwdrocarbon fractions so as to recover the valuable components from the substantially non-liqueih able gaseous components. One means has been to absorb certain of the components in an absorption oil and later recovering these components by a stripping operation. By such a process, however, all of the desirable components are not recovered. Another method has been to operate under very high pressures by means of which 20 the major portion of the hydrocarbons 'are compressed to the liquid phase and the separation made while in such condition made by fractionation. It is well known, however, that such high pressure operations are expensive to carry out. In each of these operations the major difficulty is in obtaining the proper separation from the gas components of those hydrocarbons which are on the border line between those liquefiable and those substantially non-liquefiable, and which components form a valuable part of the high gravity stabilized gasoline desired. The usual operating conditions maintained in the compression, absorption or other separation process are approximately within the critical ranges of these particular components. Therefore, any operation which permits the condensation and separation of these components without expensive or difficult processing is of great importance.

We have found a method of processing by means of which a considerable drop in operating temperatures for the purpose of cooling and condensing these hydrocarbons can be obtained cheaply, without expensive equipment, and by means of energy generated within the process itself. Operations under extremely high pressures are, of course, expensive and difiicult to carry on, both from the standpoint of equipment necessary and performance of the operation. It is desirable, therefore, to carry on the operations at a pressure as low as is practical. The other alternative is to'operateat as low temperatures as possible. However, the usual cooling medium, namely, water, is a limiting element in this re- 55 spect particularly when operating in the summer or in warm countries. Furthermore, the amount of water necessary to obtain a desired cooling effect is sometimes so great as to make the operation impractical. In certain cases, separate special refrigeration processes have been resorted to but such a method is expensive and can only be used in specialized cases where the expense is justified.

One of the primary objects of our invention is toovercome the disadvantages outlined above.

Another object of our invention is to obtain a separation and gasoline stabilizing system ca- ..pable of producing stabilized gasoline without the necessity for an external refrigeration system or the use of excessive amounts of cooling mediums.

Our system consists broadlyin separating the heavier components of the vapor mixture, which components are fractionated under pressure and in utilizing in the process the refrigerating effect obtained by the expansion of the gases separated under pressure during such fractionation.

Another object of our invention is to utilize energy'generated withina system as a cooling medium in a refinery process.

Our system, as more specifically disclosed herein, consists primarily in absorbing certain of the gases within an absorbing medium, the unab- I sorbed gases passing from the system. The absorbed gases are then removed from the rich absorption oil in a stripping tower, cooled, and a condensate returned to the top of the fractionating or distilling tower. The remaining vapors are then further cooled to separate out a second condensate which condensate is sent to a stabilizing tower to form stabilized gasoline. The cooling effect to obtain this second condensate is procured by expanding the gases obtained from the top of the stabilizing tower. The tower may be operated under higher pressure than the rest of the system. It is bymeans of the cooling effect of these expanding gases that the necessary drop in temperature is obtained in the vapors from the distillation tower and which thus permits the separation of the desirable condensate from which the stabilized gasoline is obtained. The remaining uncondensed vapors from the stripping or distillation tower are directed to a second absorption tower where they are again contacted with an absorption medium which medium may be discharged into the same stripping tower.

The second condensate which is discharged to the stabilizing tower is fractionated under pressure, the vapors passing ofi of the top of the tower being cooled to form a reflux and the uncondensed portions, after further cooling for partial condensation consisting primarily of propane, being directed to those points in the system in which the necessary cooling effect is to be obtained by their expansion. The stabilized gasoline is drawn from the bottom of the stabilizing tower.

While we have here described only the simplest outline of our process, we do not wish to be limited thereby for it will be seen from the detailed discussion and also from the drawing that there are other features of our invention.

We will refer now more particularly to th drawing of which the single figure is a diagrammatic representation of an apparatus suitable for carrying out our process and in which like reference characters denote like parts.

Reference characters I and 2 denote absorption towers through which gas is directed countercurrently to an absorbing oil medium. The tower 3 is a fractionating, distillation or stripping tower in which the absorbed, vapors are separated from the oil menstruum. The tower 4 is a fractionating orstabilizing tower into which a liquid is discharged to separate therefromcertain gaseous components and to obtain therefrom a product having a more narrow boiling point range. The absorption towers I and 2am fitted with the conventional type of trays or plates or similar equipment to facilitate absorption. For example, bellcap and downflow equipment may be used. They also may be fitted at their upper portion with mist separators 6. I

The stripping and stabilizing towers 3 and 4 are also preferably fitted with fractionating sections or plates I which also may utilize the bellcap downflow equipment These towers are also equipped with a heating means 8 which, for example, may be of steam, either in an open or closed coil. The gas and vapor mixture to be separated and recovered enters the absorption tower I through the pipe 9 and passes upwardly through the tower, the unabsorbed gases passing of! through the pipe II). The absorbing oil enters the top of the tower I through the line I I. This oil may be obtained from the bottom of the distillation or stripping tower 3 in which any previously absorbed components have been removed. There may be, of course, a pump I2 and a heat exchanger I3 in this line II in order to remove the oil'and cool it so as to make it the most effective in its absorbing operation. The rich oil after contacting with the vapors counter-currently in the tower passes from the bottom of the absorption tower I through the line l4 into a collecting drum I5 and is drawn by means of the pump I6 through the line I! and discharged into the stripping tower 3. In this tower the absorbed vapors are removed and the cycle is repeated, the stripped menstruum passing from the bottom of the tower 3 through the line II. To facilitate distillation or stripping heat may be supplied at the bottom of the tower 3 as shown at 8. Vapors pass off from the top of the stripping tower 3 through the line I8 and a portion at least is condensed in the condensing coil or heat exchanger I9, and the gaseous vapors are discharged into the separating drum 20. Reflux condensate may be returned through the line 2| and by means of the pump 22 to' the top of the stripping tower. The vapors from the separating drum 20 pass off through the line 23 and are cooled by the condenser 24. They are then given the necessary further cooling to form the desired condensate by means of the condenser 25. The cooling effect in this condenser is obtained by the expansion of a partially liquefied gas, the source of which is from the stabilizing tower as will be later described.

The cooled and condensed vapors and gases pass from the condenser 25 through the line 26 into the separating drum 21. The condensate from this separating drum passes off through the line 28 and is forced by means of the pump 29 into the stabilizing tower 4. The vapors or gases from the separating drum 21 may be sent to a compression unit which will separate a liquid and a gas. The gas may then be passed from the system and the compression liquid discharged to the stabilizing tower. The stabilized gasoline is drawn off from the bottom of the tower through the line 45 and may be cooled by means of the heat exchanger 46 and passed to storage.

Another equally suitable method, however, and the one which we have illustrated in our drawing, is to discharge the vapors from the separating drum through the line 30 into a second absorption tower and the absorbing oil which may be carried by a branch of the line II, namely I I', is discharged into the top of the tower and the rich oil removed from the lower portion of the tower through the line 3| and forced by the pump 32 into a collecting drum I5. The unabsorbed vapors pass off from the topof the absorption tower 2 through the line 33 and may pass fromthe system. All such unabsorbed gases may, of course, be used as fuel or may be otherwise treated in the refinery. The stabilizing tower 4 is operatedat pressures between 100 and 400 pounds perv square inch and preferably between 250 and 350 pounds per square inch. The condensate is forced into this tower by means of the pump 29 through the line 28. As already described, the tower is fitted with fractionating trays or plates; Heat may be supplied at the lower part of the tower as indicated at 8. The vapors passing off from the top of thetower through the line 34 are cooled andcondensed in the condenser 35 and are discharged into the separating drum 3B. Reflux condensate may be drawn from the bottom of the drum through the line 31' and forced by the pump 38 backinto the tower. The vapors from the topof the separating drum 36 are discharged through the line 39 and are further cooled while still' under pressure by means of the condenser or heat exchanger 40.

This gaseous material may consist of from 50 to propane, and is preferably cooled until it approximates F. The cooled gas under pressure is carried to the cooler or condenser 25 which acts upon the vapors remaining after the first condensation of the overhead from the stripping tower as already described. The-pressure is released on this compressed gas at the expansion valve M. The expanded gases may be discharged from the system through the line 42 in a manner similar to that already described in connection with other waste gases. The cooling effect of the expanding gas from the top of the stabilizer may also be used in other parts of the reducing valve 44. This heat exchanger, it

will be noted, operates on the stripped absorbing oil and cools it to increase its absorbingaction in the absorbing towers I and 2.

The absorbing tower I may be carried between upon the operating conditions desired.

We have found in actual practice that we have been able to obtain a cooling effect of greater than 10 F. in using our method of expanding the propane gas obtained from the stabilizing tower over that obtained 'by the most emcient' water cooling. For example, the cooling efiect obtained in the water cooled heat exchanger 24 may bring the vapors passing off from the separator 20 down to a temperature from to F. whereas, after passage through the heat exchanger 25, cooled by the expanding propane, the vapors and condensate may be cooled to a temperature between 73 and 80 F. This added cooling eifect is of tremendous significance in this operation for it should be realized that these temperatures and pressures under which the operation is carried on are approximately within the critical ranges of the hydrocarbons comprising the vapors. A

10 lowering of temperature of the vapors passing through the line 23 therefore makes the difference between a successful operation or separating out and stabilizing the vapors'and one which is unsuccessful. Further, it will be appreciated this essential cooling efiect is obtained without the use of special external refrigerating equipment or excessive use of cooling waters. As a matter of fact, it would ordinarily be impossible to get with ordinary cooling water such low cooling temperatures as we obtain except when operating under cold winter conditions.

It will be appreciated that other hookups and arrangements of our system may be made without departing from the spirit of our invention, the system described and illustrated in our drawing being for the purposes of illustration only. This illustration, of course, we have not made complete as to heat exchangers, units, control devices, or other well known plant equipment, as the disclosure has been made as simple as possible in order that the broader principles of our invention may be more clearly disclosed.

What we claim is:

1. A process for recovering valuable hydrocarbons from a gas mixture containing substantially non-con-densible gases, which comprises treating said gas mixture with an absorbing oil whereby substantially all the desired hydrocarbons are removed from said mixture together with some undesired hydrocarbon gases, vaporizing and stripping said hydrocarbons from said absorbing oil, cooling the resulting hydrocarbon vapors to a temperature somewhat above the normal temperature in an initial cooling stage, further cooling said vapors to a temperature below the normal temperature by indirect contact with expanding gases in a final cooling stage, whereby the majority of said desired hydrocarbons are condensed as a liquid, separating the condensed fraction from the uncondensed vapors and uncondensible hydrocarbon gases, forcing said condensed hydrocarbon fraction into a fractlonating zone maintained at higher pressure, fractionating said condensed hydrocarbon fraction to produce a desired liquid fraction free from uncondensible gases and an overhead fraction comprised of uncondensible gases and undesired low boiling liquid hydrocarbons, expanding said overhead liquid fraction in said final cooling stage to provide refrigeration necessary for condensing the desired hydrocarbons from the said gas mixture.

.2. The process of claim 1 wherein the said uncondensed vapors and uncondensible hydrocarbon gases separated from the condensate obtained in the final cooling stage are treated with an absorbing oil to recover valuable hydrocarbons therefrom, the unabsorbed gases are rejected from the system and the rich absorbing oil is introduced into said vaporizing and stripping step of .the process.

3. The process of claim 1 wherein the undesired low boiling liquid hydrocarbons obtained in the overhead fraction from the fractionating zone consist substantially of ethane and propane.

4. The process of claim 1 wherein the pressure maintained in said fractionating zone is between 250 and 400 pounds per square inch.

5. The process of claim 1 wherein the pressure of the gas in contact with said absorbing oil is approximately 50 pounds per square inch, the pressure of the gas in said vaporizing and stripping operation is approximately 45 pounds per square inch and the pressure in said fractionating zone is between and 400 pounds per square inch.

6. A process for recovering valuable hydrocarbons from a gas mixture containing substantially non-condensible gases, which comprises treating said gas mixture with an absorbing oil in an absorption zone maintained at a pressure of about 50 pounds per square inch, whereby substantially all the desired hydrocarbons are absorbed from said gas mixture, together with some undesired hydrocarbon gases, vaporizing and stripping said desired hydrocarbons from said absorbing oil in a stripping zone maintained at a pressure of about 45 pounds per square inch, cooling the resulting vapors and gases in an initial cooling zone to a temperature somewhat above normal available cooling temperatures, further cooling said vapors in a final cooling zone to a temperature below v the normal temperature available and below the critical temperature of ethane by indirect contact with expanding hydrocarbon gases, thereby condensing the majority of said desired hydrocarbons, separating the condensed hydrocarbons from. the uncondensed gases, forcing said condensed hydrocarbon fraction into a fractionating zone maintained at a pressure between 100 and 400 pounds per square inch, fractionating said hydrocarbon vapors to produce a desired liquid fraction free from uncondensible gases and an overhead fraction comprised of uncondensible gases and undesired low boiling liquid hydrocarbons, expanding said overhead fraction in said final cooling zone to cool the gases therein below the critical temperature of ethane.

JOSEPH K. ROBERTS. PERCY c. KEITH, JR. 

